1. Field of the Invention
The present invention relates to a chiral compound, and in particular relates to a photosensitive chiral compound having high helical twisting power and a liquid crystal composition containing the same.
2. Description of the Related Art
A cholesteric liquid crystal display can reflect environmental light without requiring a backlight module and a polarizer film, and therefore can provide reduced power consumption when compared with the conventional transmissive liquid crystal displays. Since the cholestric liquid crystal display can reflect lights of various wavelengths, color images can be displayed without a color filter.
A cholesteric liquid crystal, also called a chiral nematic liquid crystal material, is composed of a nematic liquid crystal and chiral dopant. The nematic liquid crystal exhibits higher helical twisting power than that of twisted nematic (TN) or super twisted nematic (STN) liquid crystals due to the presence of the chiral dopant.
The pitch (p) of the cholesteric liquid crystal is determined by the concentration (c) and the helical twisting power (HTP) of the chiral dopant, which can be expressed by the formula:p=1/(HTP·C)
The reflected wavelength of the cholesteric liquid crystal is determined by the pitch thereof, while the pitch is controlled by the helical twisting power and the concentration of the chiral dopant. Therefore, under a given value of pitch, the higher the helical twisting power, the lower the required concentration of the chiral dopant. If the chiral dopant has poor solubility in the liquid crystal host (multi-layer nematic liquid crystal), it will be difficult to tailor the liquid crystal formulation.
It is desirable to use a photosensitive chiral compound having high helical twisting power in optical writing cholesteric liquid crystal displays. The conformation of the chiral molecules in the liquid crystal can be altered by illumination to result in the change of the helical twisting power, which in turn, changes the pitch of the cholestric liquid crystal to obtain a different color of reflected light. However, if the chiral compound has a inherent color (such as an orange azo molecule), its addition to the liquid crystal host will interfere with color reflection, making the chiral compound not suitable for use as the liquid crystal formulation in an optical writing cholestric liquid crystal device.
Accordingly, it is desirable to provide photosensitive chiral compounds having high helical twisting power, good solubility in a liquid crystal formulation, and colorless properties, to be used in an optical writing cholestric liquid crystal device. Furthermore, the liquid crystal composition preferably should have high color purity and stable temperature dependence.